The present invention relates to a safety switching device for the failsafe shutdown of an electrical load, and, in particular, for the failsafe shutdown of a machine or a machine installation which poses a hazard to people.
Safety switching devices usually have a number of terminals or device connectors to which so-called signaling devices, on the one hand, and actuators on the other hand are connected. Typical signaling devices for safety switching devices are emergency-off push buttons, guard-door switches, two-hand operated switches, light barriers and various sensors which provide safety-related signals with respect to a monitored machine or machine installation. The safety switching device monitors the safety-related signals from the signaling devices, evaluates them and generates in dependence thereon control signals for actuators which are designed for shutting down the hazardous machine or machine installation in a failsafe manner. In addition, the safety switching devices often generate output signals by means of which the operating state of a simple mechanical signaling device such as, for instance, an emergency-off push button is monitored. For example, an output signal generated by the safety switching device might be fed back in a loop via electrical contacts of an emergency-off push button to an input of the safety switching device, and the safety switching device shuts down the machine or machine installation if the fed back output signal is not detected. In contrast, light barriers or intelligent sensors usually generate their own output signals which can be received at an input of the safety switching device and evaluated. In such a case, the safety switching device does not need to provide an output signal for monitoring the light barrier even though this is still possible for certain light barriers, for instance for diagnostic purposes.
Depending on the number and type of safety functions which have to be monitored at a machine or machine installation, a suitable safety switching device needs a certain number of inputs and outputs for connecting signaling devices and actuators. The number of inputs and outputs increases when a high safety category is required for safeguarding a hazardous machine or machine installation. In such cases, the signaling device, the safety switching device and the actuators are typically connected via redundant lines so that the number of required inputs and outputs is typically doubled.
On the other hand, there is a desire to make a safety switching device as compact and small as possible since the installation space in a switchgear cabinet or the like is often limited. There are devices, therefore, in which terminals for connecting signaling devices and actuators can be configured so that the function of the terminal can be different depending on the respective application. For example, the assignee has sold decentralized I/O assemblies called PSS SB DI16 for a decentralized safety control system in which different types of output signals could be provided at certain connecting terminals. In particular, steady-state output signals or clocked output signals having different clock periods could selectively be provided at certain output terminals. Accordingly, it was possible to reduce the number of connecting terminals and thus also the housing size of the devices. However, the configurable connecting terminals were not available for all safety functions, especially not for a change from an input function to an output function.
EP 1 347 388 B1 discloses a bus coupler for connecting signaling devices and actuators to a field bus system. To allow a flexible connection of safety sensors and standard sensors, EP 1 347 388 B1 proposes that different functional characteristics from a set of predefined functional characteristics can be applied to individual connecting pins of the plugs. In one exemplary embodiment, the plug has five contact elements, at least some of which can be selectively used as input or as ground terminal. However, circuit details which allow such a variable use of a contact element are not described in this document.
DE 199 62 497 A1 discloses a safety switching device having an input circuit for receiving an input signal from a signaling device which signals a safety-related state, having an output circuit for outputting an output signal and having switching elements which are designed for interrupting a current supply path to an electrical load. An evaluation and control unit actuates the switching elements in dependence on the input signal. The input circuit comprises an optocoupler which combines a clock signal generated by the safety switching device with the input signal from the signaling device. The periodic clock signal controls a light-emitting diode of the optocoupler whilst the signal from the signaling device is conducted via the collector-emitter path of a light-sensitive transistor. Combining the input signal from the signaling device with the clock signal from the safety switching device results in the signal from the signaling device being modulated with the clock signal. As a consequence, the combined signal has periodic signal changes which allow the safety switching device internal functional testing of its signal paths even if the signal from the signaling device remains static in one signal state over relatively long periods.
DE 102 11 099 A1 discloses a device for actuating an electrical load, wherein a control transistor is connected via a line to a port of a control and evaluation unit. An output of the control transistor is connected to the control input of a driver transistor. At the output of the driver transistor, the electrical load is arranged. In addition, a voltage feedback unit is provided which couples the electrical voltage at the output of the driver transistor back to the line between the port of the control and evaluation unit and the control transistor. The port of the control and evaluation unit can be configured as output for outputting the control signal or as input for sensing a signal present on the line.